Infra-red transparant materials

ABSTRACT

Material transparent at infra red wavelengths, 3-5 and 8-14 μm is formed of ZrN, YN, CeN, TLN, or EuN. The material can be used as a self supporting material or as a coating on substrates such as infra red transparent material e.g. Ge, ZnS, ZnSe, As 2  S 3 , As 2  Se 3 , optically transparent materials e.g. sodium glass, or reflecting surfaces such as metal surfaces, e.g. Al or silvered surfaces. For some substrates e.g. Ge a thin, e.g. 10-1,000 Angstrom bonding layer may be used to improve adhesion. Bonding layers may be Ge, Si, Ge x  C 1-x , Si x  C 1-x  (0&lt;x&lt;1). The coating may be produced by sputtering in a glow discharge chamber using Ar and N gases.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to infra red transparent materials. Suchmaterials are useful as windows and lenses for thermal imaging systems,either as a coating or as a self supporting layer.

2. Discussion of Prior Art

Materials commonly used are germanium, zinc sulphide, and zinc selenide.All are relatively soft and therefore easily damaged. One method ofprotecting these soft materials is to coat them with a hard material.The coating most successful to date is a hard carbon that is diamondlike in its hardness. This is described in GB 2,082,562 B. Disadvantagesof this material are the interstitial graphitic inclusions which limitthe optical transmission and the internal strain which prevents layersthicker than about 1 um being grown. An alternative coating of hardcarbon includes a small amount of germanium to relieve stress and allowthicker coatings to be made; this is described in GB 2,129,833 A and itsdivisional 85 24,696.

Ideally a hard coating should be transparent in the 3-5 and 8-14 μminfra red wavebands, and also in the visible band i.e. about 0.4 to 0.7μm. It is further desirable that the coating is stable and transparentat high temperatures so that it may be used as a window for hightemperature processes. A disadvantage of the hard carbon, and hardgermanium carbon coatings is their high temperature performance. Onheating to say 600° C. the carbon changes to a graphitic form which isabsorbing to infra red radiation. These materials are also absorbing tovisible light in useful thicknesses.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a material that is infrared transparent over a wide band of wavelengths and elevatedtemperatures.

According to this invention an infra red transparent material is formedof Zirconium nitride or Yttrium nitride also Cerium or Thorium orEuropium nitride.

The material my be used as a coating on infra red transparent substratematerial such as Ge, ZnS, ZnSe, AsS₃, AsSe₃ ; optically transparentmaterial such as sodium, silica or lead glass; any suitable metal suchas Cu, Al, alloys of Al, alloys of Fe or silvered surfaces; or as a selfsupporting layer in which case the supporting substrate is removed e.g.by etching. These infra red transparent materials Ge, etc., aretransparent in the 1.9-2.7, 3-5 and 8-14 μm wavebands. ZnS, ZnSe, As₂S₃, As₂ Se₃ are also partly transmissive in the visible waveband. Glassis transparent up to about 2 5 μm i.e. the visible and near infra redwaveband. The coating is hard and may therefore be used for its abrasionresistant properties for example on infra red windows and lenses.Alternatively it may be used on metal to maintain a highly polishedsurface on components such as turbine blades and ductings. A further useon metals is to provide a hard wear resistant coating on cutting toolsused on lathes etc.

The material may be formed by reactive sputtering using a target of ZrY, Ce, Eu or Th in a DC or RF glow discharge of the gases Ar and N.

According to this invention an optical component comprises a transparentsubstrate coated with a transparent thin layer of ZrN, YN, CeN, EuN orThN, the component being transparent in either or both the infra red(1.9-2.7, 3-5 and 8-14 μm) and visible band (0.4 to 0.7 μm) ofwavelengths.

According to an aspect of this invention a machine tool cutting tip,turbine or pump blade is coated with an abrasion resistant coating ofZrN, YN, CeN, ThN or EuN.

According to another aspect of this invention the reflecting surfaces ofa direct view thermal imager are coated with a thin layer of ZrN, YN,CeN, Thn or EuN.

According to this invention a method of producing Zirconium, Yttrium,Cerium, Thorium or Europium nitride comprises the steps:

providing an anode and a cathode inside a vacuum chamber,

arranging a target of Zr, Y, Ce, Th or Eu on the cathode,

mounting a substrate to be coated opposite and spaced from the cathode,

maintaining the substrate at a desired temperature of between 300° and600° C.,

flowing gases of argon and nitrogen through the chamber whilstmaintaining a reduced pressure inside the chamber,

providing a glow discharge plasma inside the chamber between the anodeand cathode whereby argon ions sputter off material from the target tocombine with nitrogen on the substrate and form the desired coating.

The glow discharge may be provided by an R.F. or a D.C. electric field.Enhanced deposition rate may be provided by magnetron sputtering.

The substrate to be coated may be mounted directly on the anode orspaced therefrom.

Alternatively the coatings may be grown by molecular beam epitaxy (MBE)growth apparatus, vapour phase epitaxial growth apparatus, or by aceramic process. In this latter case a powder of Zr, Ce, Th, or Eunitride is formed and milled to the required particle size, then pressedinto a blank of the required shape and fired at an elevated temperatureuntil a ceramic blank is formed.

Care must be taken in forming the coating otherwise a dark absorbinglayer is grown.

BRIEF DISCUSSION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings of which:

FIG. 1 is a cross sectional view of glow discharge apparatus,

FIG. 2 is a sectional view of a component coated in the apparatus ofFIG. 1,

FIGS. 3(a), (b), 4(a), (b) are graphs of transmission against wavelengthfor the component of FIG. 2.

FIG. 5 is a diagrammatic view of a pyro-electric vidicon camera.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

As shown in FIG. 1 glow discharge apparatus comprises a vacuum tightchamber 1 containing an anode plate 2 and a cathode plate 3. The anode 2is heated by a heater 4 and carries a substrate 5 to be coated. Examplesof substrate materials are Ge, ZnS, Zn Se, and metals such as Cu, Al,silvered surfaces, and various alloys and steels. Both the chamber 1 andanode 2 are connected to earth. The cathode 3 carries a plate of targetmaterial 6, e.g. Zirconium (Zr) or Yttrium (Y), and is connected to anRF source 7 via a capacitor 8. Negative voltage developed at the cathodeis measured by a D.C. voltmeter 9. Gas bottles 10, 11 supply nitrogenand argon through valves 12, 13, 14 into the chamber 1. A vacuum pump 15removes gases from inside the chamber 1 via a restrictor valve 16.

Operation to grow a Zr, or Y nitride layer 17 transparent to a wide bandof wavelengths, including the infra red and visible, is as follows: Thetarget 6 and substrate 5 are mounted as shown on the cathode 3 and anode2 respectively. The anode 2 and substrate 5 temperatures are raised toabout 500° to 550° C. Other temperatures may be used. However, belowabout 500° C. the grown layer becomes increasingly absorbing. Aboveabout 600° C. there are practical difficulties in providing ananode/substrate holder and reliable resistance heaters. The pump 15 isoperated to drop the pressure to about 30 millitorr whilst nitrogen andargon gasses are admitted from the bottles 10, 11 and an RF D.C. bias ofabout -1 kvolts is applied to the cathode. This produces a plasmadischarge between the anode 2 and cathode 3. Argon ions strike thetarget and remove atoms of the target material which combine withnitrogen in the plasma to deposit as a layer 17 of zirconium nitride oryttrium nitride on the substrate 5. For correct deposition parametersthe layer is transparent to a wide band of wavelengths e.g. 0.4-16 μm.

Ce, Th or Eu nitride coating layers are grown in a similar manner.

When growing Zr nitride the percentage of N to Ar is about 50%. For Ynitride the percentage of N to Ar is about 1%. Deposition rates aretypically 0.2 μm/hour for ZrN and 0.4 μm/hour for YN. These rates nay beincreased by magnetron sputtering techniques.

Prior to coating the substrates may be cleaned for example by Ar ionbombardment. This my be achieved by mounting the substrate on thecathode 3, admitting Ar into the chamber whilst reducing the pressure toabout 20 millitorr. A glow discharge is initiated and maintained forabout 5 minutes. A similar cleaning process may be made to the target 6.

Coatings of ZrN and YN have excellent adhesion to most metals, e.g. Al,Duraluminium, Cu, stainless steel and Ag. The adhesion on Ge, ZnSe, andZnS is good but less than for the metals. To improve adhesion on Ge,ZnSe, and ZnS, a very thin bonding layer (less than 0.5 um) of Ge_(x)C_(1-x) (0<x<1) may be deposited from a vapour of Ge and C. Typicallyonly a few Angstroms thick layer is needed. For example the bondinglayer may be 10 to 1,000 typically 100 Angstroms thick. Being so thinthe bonding layer has negligible effect on transmission at anywavelength. The Ge and C vapour may be produced in the apparatus of FIG.1 using a glow discharge to dissociate germane and hydrocarbon gases,without sputtering from a Zr or Y target. Other bonding materials areGe, Si, and Si_(x) C_(1-x) produced as for GeC. Silicon alone, withcarbon, or as an oxide may be used. This can be deposited using a silanefeedstock.

FIGS. 3(a), (b) show the transmittance values for a Ge substrate and fora Ge substrate coated with 0.5 um of ZrN, the coated substrate being theupper two traces (carried out with slightly different apparatussettings). FIGS. 4(a), (b) show transmittance values for Ge and Gecoated with a 1 um thick layer of YN. Both FIGS. 3 and 4 show excellenttransmittance from 2.5 um out to 12 um and 14 um for ZrN and YNrespectively.

Coating thickness for anti-reflection proporties can be calculated asfollows:

    2n.sub.1 d=λ/2

where

n₁ is refractive index of coating

d is coating thickness

λ is wavelength of radiation concerned.

For good matching n₁ =/n_(o) ×n₂

where n_(o) and n₂ are the refractive indexes of the material eitherside of the coatings. For air n_(o) =1, for bulk Ge n₂ =4.

The refractive index was found to be 2.1 for both ZrN and YN which isideal for use as an antireflection coating on Ge lenses. For use in 8-14μm thermal imagers the antireflection coating would be 1.2 μm thick fora λ/4 thickness at 10 μm wavelength.

The coatings were found to be chemically inert and very hard, having ahardness value >2000 Knoop. This approaches that of diamond-like carboncoatings of the prior art. Unlike these diamond-like coating the ZrN andYN coatings are stable at temperatures in excess of 500° C. for longperiods.

These properties make the coating useful as windows on tanks and othervehicles where the combination of windsreen wipers and sand make itessential that very hard coatings are used. Behind these windows thermalimagers are arranged to view the thermal scene. Prior to theintroduction of hard carbon coatings the lifetime of such windows wasvery short. An advantage of coatings of the present invention over hardcarbon coatings is their optical transparancy. This allows both thermaland optical imaging systems to be arranged behind the windows ofoptically and infrared transmitting material, e.g. ZnS, ZnSe As₂ S₃, As₂Se₃ window.

The refractive index of ZrN, YN at about 2.1 is similar to that ofZnS,ZnSe, and sodium glasses. Thus coatings directly on such substratesare protective without being anti-reflective. To provide anti-reflectiveproperties a multi coated layer must be used. For example on ZnS or ZnSesubstrates a GeC λ/4 layer with an n≅3 may be used directly on thesubstrate. As taught in GB 2,129,833 the value of n is variable with therati of Ge to C. Next a layer of GeC with a graded refractive index(effective n=4) is used by varying the Ge:C ratio. The final layer is ofa λ/4 thickness (e.g. 1.2 μm at 10 μm wavelength) of ZnN, or YN.

Alternatively on ZnS, ZnSe substrates a thick coating, e.g. up to 20 umor more, may be used. This thick coating gives added mechanicalprotection. A final anti-reflection coating of ThF may be deposited.This is not very hard but for some applications gives an adequateprotection.

Another use of the coatings of the present invention is as a frontcoating on Ge lenses. FIG. 5 shows a pyroelectric vidicon camera used byfire fighting services to see through smoke inside burning buildings.These known cameras 20 have a front Ge lens 21 plus other smaller lenses22 focussing the thermal scene onto a pyroelectric detector tube 23.Output from the tube 23 is used by control circuits 24 to modulate acathode ray tube (CRT) 25 and form a visible display 26 of the thermalscene. The front lens 21 is coated with ZrN or YN so that dirt is easilyremoved without damaging the soft Ge material.

Similarly the front lens of optical cameras used in surveying sewers etcmay be coated with ZrN or YN to prevent damage.

The high temperature stability enables the material to be used onwindows subject to adverse conditions such as in furnaces etc.

Another use of the invention is in direct view thermal imagers. Theseare known imagers, e.g. G.B. 2,291,196 A, having rotating polygonsand/or flapping mirrors to sweep scan the image of a thermal scene ontoan infra red detector. Output from the detector modulates the lightoutput from a lamp such as a light emitting diode (LED). The rotatingpolygons and/or flapping mirror also scans the LED into an eye piece forobservation by an operator. The scanning mechanism is thus used to scaninfra red and visible light.

A problem with rotating polygons is tarnishing of the highly polishedreflecting surfaces. One solution to this problem is disclosed in G.B.2,067,304. A thin layer of infra red transparent amorphous carbon isdeposited on the reflecting surfaces. Such a solution is only useful instandard imagers where the detector output modulates a C.R.T. display.The hard carbon is opaque to visible light. Therefore it cannot be usedin direct view thermal imagers.

However this problem can be overcome by use of the present invention.Reflecting surfaces are protected by a thin coating of ZnN, YN, CeN, ThNor EuN, transparent at infra red and optical wavelengths. The coatingmay be less than 1 μm--typically about 0.1 to 0.2 μm thick.

Self supporting thick, e.g. 10 to 1000 μm or more, layers may be growne.g. on Al substrates, and the substrate subsequently removed by anetchant such as nitic acid, or hydrochloric acid.

We claim:
 1. A method of producing an infrared and visible lighttransparent coating comprising the steps of:providing an anode and acathode inside a vacuum chamber, arranged a target of at least one ofZr, Y, Ce, Th and Eu on the cathode, mounting a substrate to be coatedopposite and spaced from the cathode, maintaining the substrate at adesired temperature of between 500° and 600° C., flowing gases of argonand nitrogen through the chamber whilst maintaining a reduced pressureinside the chamber, providing a glow discharge plasma inside the chamberbetween the anode and cathode whereby argon ions sputter off materialfrom the target to combine with nitrogen on the substrate and form thedesired coating transparent to visible and infra red wavelength of 3-5and 8-14 um.
 2. An improved infrared and optically transparent coatingfor a substrate, said coating transparent to visible and infraredradiation having a wavelength between 0.4 and 16 μm, said coatingcomprising a layer of a nitride of one of Zirconium, Yttrium, Cerium,Thorium and Europium, wherein said substrate is one of germanium, zincsulphide, zinc selenide, arsenic sulphide, arsenic selenide, copper,aluminum, sodium glass, silver and steel.
 3. The coating of claim 2,wherein said coating further includes a bonding layer between nitridelayer and said substrate, said bonding layer comprising an infrared andvisible light transparent layer less than 0.5 μm thick.
 4. The coatingaccording to claim 3, wherein said bonding layer is one of Ge, Si,Ge_(x) C_(1-x), Si_(x) C_(1-x) where 0<x<1.
 5. The coating according toclaim 2, wherein said coating is an anti-reflective coating for awavelength λ wherein said nitride layer has a refractive index n₁ andhas a thickness d, where d equals λ/4n₁.
 6. An infrared and visiblelight transparent window, said window including an infrared and visiblelight transparent substrate, said window provided by the stepsof:providing an anode and a cathode inside a vacuum chamber; arranging atarget of at least one of Zr, Y, Ce, Th and Eu on said cathode; mountingsaid substrate opposite and spaced from said cathode; maintaining saidsubstrate at a desired temperature of between 500° C. and 600° C.;flowing gases of argon and nitrogen through said vacuum chamber whilemaintaining a reduced pressure inside said chamber; and providing a glowdischarge plasma inside said chamber between said anode and saidcathode, whereby argon ions sputter off material from said target tocombine with nitrogen on said substrate forming said desired windowtransparent to visible and infrared radiation.
 7. The infrared andvisible light transparent window according to claim 6 wherein prior tosaid second providing step, there is the additional step of growing abonding layer of one of Ge, Si, Ge_(x) C_(1-x) and Si_(x) C_(1-x) of athickness less than 0.5 μm where 0<x<1.
 8. The infrared and visiblelight transparent window of claim 6 wherein said substrate is sodiumglass.
 9. The infrared and visible light transparent window according toclaim 6 wherein said window includes an anti-reflective coating for awavelength λ, said coating having a refractive index n₁ and a thicknessd, where d equals λ/4n₁.
 10. An infrared and visible light transparentcoating for a substrate, said coating provided by the steps of:providingan anode and a cathode inside a vacuum chamber; arranging a target of atleast one of Zr, Y, Ce, Th and Eu on said cathode; mounting saidsubstrate to be coated opposite and spaced from said cathode;maintaining said substrate at a desired temperature of between 500° C.and 600° C.; flowing gases of argon and nitrogen through said vacuumchamber while maintaining a reduced pressure inside said chamber; andproviding a glow discharge plasma inside said chamber between said anodeand said cathode, whereby argon ions sputter off material from saidtarget to combine with nitrogen on said substrate forming said desiredcoating transparent to visible and infrared radiation.
 11. The infraredand visible light transparent coating according to claim 10, whereinsaid substrate is one of germanium, zinc sulphide, zinc selenide,arsenic sulphide, arsenic selenide, copper, aluminum, silver and steel.12. The infrared and visible light transparent coating according toclaim 10, wherein said substrate is a metallic reflecting substrate andsaid coating is a layer less than 1 μm.
 13. A method of producing aninfrared and visible light transparent coating on a non-metallicsubstrate comprising the steps of:providing an anode and a cathodeinside a vacuum chamber, arranging a target of at least one of Zr, Y,Ce, Th and Eu on the cathode, mounting said non-metallic substrate to becoated opposite and spaced from the cathode, maintaining the substrateat a desired temperature of between 500° to 600° C., flowing gases ofargon and nitrogen through the chamber whilst maintaining a reducedpressure inside the chamber, providing a glow discharge plasma insidethe chamber between the anode and cathode whereby argon ions sputter offmaterial from the target to combine with nitrogen on the substrate andform the desired coating transparent to visible and infra red wavelengthof 3-5 and 8-14 μm.
 14. The method of claim 1 wherein, prior to growingthe coating, providing the additional step of growing a bonding layer ofone of Ge, Si, Ge_(x) C_(1-x), and Si_(x) C_(1-x) of thickness less than0.5 μm, where 0<x<1.
 15. The method of claim 13, wherein, prior togrowing the coating, providing the additional step of growing a bondinglayer of one of Ge, Si, Ge_(x) C_(1-x), and Si_(x) C_(1-x) of thicknessless than 0.5 μm, where 0<x<1.
 16. An improved infrared and opticallytransparent coating for a non-metallic substrate, said coatingtransparent to visible and infrared radiation having a wavelengthbetween 0.4 and 16 μm, said coating comprising a layer of a nitride ofone of Zirconium, Yttrium, Cerium, Thorium and Europium, wherein saidsubstrate is one of germanium, zinc sulphide, zinc selenide, arsenicsulphide, arsenic selenide, and sodium glass.
 17. The coating of claim16, wherein said coating further includes a bonding layer between saidnitride layer and said substrate, said bonding layer comprising aninfrared and visible light transparent layer less than 0.5 μm thick. 18.The coating according to claim 17, wherein said bonding layer is one ofGe, Si, Ge_(x) C_(1-x), Si_(x) C_(1-x) where 0<x<1.
 19. The coatingaccording to claim 16, wherein said coating is an anti-reflectivecoating for a wavelength λ wherein said nitride layer has a refractiveindex n₁ and has a thickness d, where d equals λ/4n₁.
 20. An infraredand visible light transparent coating for a non-metallic lighttransmissive substrate, said coating provided by the steps of:providingan anode and a cathode inside a vacuum chamber; arranging a target of atleast one of Zr, Y, Ce, Th and Eu on said cathode; mounting saidnon-metallic light transmissive substrate to be coated opposite andspaced from said cathode; maintaining said non-metallic lighttransmissive substrate at a desired temperature of between 500° and 600°C.; flowing gases of argon and nitrogen through said vacuum chamberwhile maintaining a reduced pressure inside said chamber; and providinga glow discharge plasma inside said chamber between said anode and saidcathode, whereby argon ions sputter off material from said target tocombine with nitrogen on said substrate forming said desired coatingtransparent to visible and infrared radiation.